1. Field of the Invention
The present invention relates to a spark plug for use in internal combustion engines.
2. Description of the Related Art
As the performance of recent automotive and other internal combustion engines has improved, the temperature of spark plugs used to start the engines has increased. With increased spark plug temperatures, the spark gap formed between electrodes tend to be consumed at an accelerated rate and the endurance of the spark plug is shortened accordingly. In order to ensure corrosion resistance at high temperatures, spark plug electrodes are often made of Ni alloys such as Inconel. However, Ni alloys are generally low in thermal conductivity and permit heat dissipation at such a slow rate that the electrode temperature is prone to rise unduly during high speed driving and other operations. In order to solve this problem, a spark plug has been commercialized, in which the heat dissipation and endurance of the electrode are improved by using a core electrode member made of a high heat conducting core metal, such as a Cu-based metal.
The behavior of thermal conduction in the radial direction of an electrode in a spark plug 200 is shown in FIG. 13A. A temperature gradient is formed from the peripheral surface P1 of the electrode 200 (which may be regarded as the "heat input side") to the center. This gradient provides a driving force for the progress of heat conduction. The electrode 200 is structured such that a high heat conducting core metal 202, which serves to accelerate heat dissipation, is located in the central area of the electrode 200. Externally applied heat Q is unable to flow into the high heat conducting core metal 202 until after it passes through an sheath portion 201, which has a comparatively small heat transfer coefficient. In other words, the heat transfer through the sheath portion 201 is a rate limiting step in the behavior of heat dissipation under consideration. If the thickness of the sheath portion 201 is excessive, the heat flux through the core member 202 is reduced as shown in FIG. 13B, and the heat dissipation that can be achieved is not as great as intended. Therefore, in order to ensure effective heat dissipation, the relative thickness of the sheath portion 201 as compared to the core member 202 must be reduced. Conversely stated, the radial dimension of the high heat conducting core metal 202 has to be significantly increased.
However, if the dimension of the core member 202 is made too large, an elevation in the electrode temperature causes the production of a higher level of thermal stress due to the difference in linear expansions between the sheath portion 201 and the core member 202. This may potentially lead to interlaminar cracking and expansion electrode problems. These problems are most likely to occur in direct-injected gasoline engines and other engines of a type in which the firing portions of a spark plug project into the combustion chamber resulting in a considerably high electrode temperature. Since the increase in the dimension of the core member 202 is limited to a certain degree by the generation of thermal stress, the desired improvement in heat dissipation has not been achieved to the fullest extent.